Notice that at fully supersonic speed, the shocks (the darker lines from the T-38 that angle slightly to the right as they go above and below the vehicle) persist as they propagate away from the vehicle and the nose shock stretches farther away rear shocks. (The circular "banding" lines that bend to the left are an artifact from the light source, the sun, which were not completely filtered out in this picture during in-flight schlieren image processing.) The stretching occurs because compressed air from the nose of the vehicle reduces the local Mach number somewhat, which reduces its Mach angle relative to the higher Mach number from the expanded air above the wing. Farther from the vehicle, this propagation angle variation between the predominately high pressure toward the front of the vehicle and predominately low pressure toward the rear, will result in shock coalescence into just one front and one rear shock of the classic N-wave shape. The shock waves from high altitude stretch to double the vehicle length and more by the time they reach the ground.
The stretching and coalescence of shock waves as they propagate is called "aging." Understanding aging is the key toward understanding how to create shaped and minimized sonic booms.